Mix together all ingredients except margarine and cracker crumbs and put in casserole dish; top with cracker crumbs, browned in 1 stick of margarine. Bake for 45 minutes at 350. White sauce: Melt margarine in saucepan over low heat. Blend in flour, salt . Add milk all at once. Cook quickly, stirring constantly, until mixture thickens and bubbles. Remove from heat when it bubbles.